A while ago I made GroupMe Analysis, which is a bot that you can add to your group, and then see interesting facts about it, including number of comments, likes, and a person-by-person breakdown. However, I couldn't find any tool to download an entire GroupMe that wasn't just making a CSV file or something boring like that. So I wrote a Python file that will take in an access token from GroupMe and download an entire conversation, attachments and all, for permanent offline viewing.
As you can see, it looks relatively close to the original conversation, which was my goal with the program. You can scroll back and it's as if the entire conversation were there, even though it's just an HTML file generated by the Python script. You can download the script yourself and run it by following the instructions by checking it out on my Github here.